Apparatus for the extrusion of tubular thermo-plastic film

ABSTRACT

The present invention relates to a method and apparatus for the extrusion of tubular thermoplastic film which comprises extruding a melt of thermoplastic through an annular orifice to form a bubble or tube and while the tube is still in a semimolten condition, passing the tube through an extrusion cooling and shaping means having a plurality of multi-perforated air rings shaped in congruance to the desired shape of the inflating tube. This method allows increased production speeds and minimized gauge variation while maintaining bubble stability.

RELATED U.S. APPLICATION

This is a Division of application Ser. No. 467,187, filed May 6, 1974,now U.S. Pat. No. 3,976,732.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to an apparatus and method for producingblown tubular films from thermoplastic materials. More particularly,this invention relates to improved tube forming and cooling procedureswherein a blown tube is formed by differential air pressure produced bythe action of high velocity cooling air conducted toward and along theexternal surfaces of the advancing tube as it is being formed.

(2) Description of the Prior Art

Generally, the prior art technique for forming tubular thermoplasticfilms comprises continuously extruding a melt of a thermoplasticmaterial through an annular orifice, applying internal fluid pressure tothe tube thus formed and shape expanding the tube and reducing the wallthickness thereof to appropriate dimensions while cooling andsolidifying the extruded thermoplastic. Thereafter, the formed tubing iscollapsed by passing it through the nip formed by a pair ofcounter-rotating pinch rolls. The flattened tubing may be subsequentlypassed to a wind up station, or on to further processing such asbag-making operations for example.

Although useful tubing has been commercially prepared utilizing thismethod, under certain circumstances, such a product may have anundesirable gauge non-uniformity, i.e., the thickness of the film is notuniform. Such non-uniform wall thickness results in, for a given averagethickness, low gauge points which introduce weak areas in the film.Also, gauge variation results in an uneven, humped roll of film uponwinding of the flattened tubing. In addition to the unsightly appearanceof such rolls, when the film on such rolls is unwound, it does not lieflat and thus requires special precautions in the printing, conversionand other uses thereof.

One of the major problems in this art is to rapidly cool the extrudedbubble of thermoplastic material. Production rate for any given tube(bubble) size is limited by the character of the bubble being extruded.Thus, under a given set of operating conditions, increasing extruderoutput will cause the thermoplastic to be formed into the tube at ahigher rate but since the heat exchange character of the system will nothave changed, it will also cause a rise in the height of the frost line(that is the line where the extruded tube turns from molten to solidcharacter). This in turn causes an increase in the instability of theextruded bubble because its unsupported molten length has become toolong. Supporting the film bubble in general permits increased coolingair impingement and therefore increased extrusion speeds.

SUMMARY OF THE INVENTION

The present invention comprises a method and apparatus which eithereliminates or substantially reduces the prior art difficulties in theproduction of a thermoplastic tube at a relatively high rate of speedhaving minimal gauge variations. A method and apparatus is provided forthe extrusion of film tubes comprising molten thermoplastic resinthrough an annular die orifice in the form of a tube, inflating the tubewith air and cooling the inflated tube to solidify the moltenthermoplastic. The tube is cooled by passing it through an openingformed by a plurality of superposed circular members being supplied withair under pressure for inpingement action on the sides of the advancingtube. Superposed plenum chambers surround each circular member to supplythe member with air. Each of said members is defined by a ring elementhaving a pair of diverging aperture rows, the ring elements beingdetachably secured to the plenum chambers. Air is supplied to the plenumchambers from a plurality of circular inlet conduits located at the edgeof the chamber. The inlet conduits are located so that when two or moreseparate chambers are superposed upon one another, the individual inletconduits form a common channel which carries air under pressure from anoutside source of supply to the respective plenum chambers. The circularair inlet conduits of each plenum chamber are characterized by having araised circular boss upon their upper and lower peripheries to maintainadjacent plenum chambers in spaced-apart relation. The spacing is topermit the air which has impinged the tubing to escape from the system.More particularly, the present invention relates to an apparatus forforming tubular thermoplastic film which comprises a conventionalextruder and annular extruder die means which are employed incombination to extrude a seamless tube of thermoplastic film. A pair oftake off, pinch or pull rollers are spaced downstream from the annularextruder die and they are employed to flatten the extruded tubing aswell as draw it away from the annular die means. A fluid such as air isintroduced under pressure into the extruded tube to biaxially expand thetube wall while it is still in a semi-molten or heat softened conditionwhereby the wall thickness of the tube is reduced. A configurationimposition assembly is disposed externally and concentric to saidextruded tubing downstream from the die. This assembly comprises aplurality of spaced-apart rows of pairs of diverging aperture elementswhich face towards the external surface of the tube. A fluid such as airis forced through the descrete apertures toward the tubing and the airresultant flow dynamically creates a suction or vacuum which draws theextruded tubing toward the inside surface of the assembly. The shapeimposition assembly is further characterized by having a plurality ofvertically spaced-apart individual plenum chambers. The aperture rowmeans referred to hereinabove are detachable from the plenum chambers.The plenum chambers are so formed that a common chamber results upon thesuperimposition of the individual plenum chamber so that such a commonchamber may be employed for feeding air under pressure into each of theindividual plenum chambers so superimposed upon one another.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front elevation, in section, of a typical tubular extrusionapparatus embodying the features of the present invention.

FIG. 2 is a perspective view of one of the air ring elements of thepresent invention, a plurality of which are shown in section in FIG. 1.

FIG. 3 is an overhead planar view of the internal structure andconfiguration of the air ring element shown in FIG. 3.

FIG. 4 is a cross sectional view taken on line 4--4 of FIG. 2.

FIG. 5 is an enlarged fragmentary view of a portion of the apparatusshown in FIG. 1.

DESCRIPTION OF SPECIFIC EMBODIMENTS

Inherent in all extrusion dies there is some degree of error in thethickness of molten film exiting around the periphery of the dieorifice. This is caused by the difficulty in achieving an absolutelyuniform orifice between adjacent die lips.

In conventional processes this thickness error in the molten orsemi-molten tube leaving the die normally results in a thickness error,i.e. gauge variation, in the final film. The relationship of film gaugeprofile to die error is a complex function of cooling rates andviscosities in the molten film as it is being stretched and cooled. Ithas been found that imposition of a particular shape to the molten filmbubble permits control of these various factors so that they inter-reactin such a way as to substantially cancel out gauge errors introduced byvirtue of the aforedescribed extrusion die non-uniformity.

It has been found that if the bubble is caused to blow-up to itsenlarged diameter close to the die, there will be a direct relationshipbetween the die irregularities and the irregularities in the resultantfilm; that is, where the molten film leaving the die is thickest, thefinal film will be thickest and vice-versa. Conversely, if the point ofblow-up is moved further from the die there will be an inverserelationship, i.e. a thin area leaving the die will result in a thickarea in the final film product. By choosing the proper shape, it ispossible to effect a cancellation or substitute cancellation of suchgauge errors.

In a conventional process, such control is not readily achievable, if atall, whereas practice of the present invention provides the necessarycontrol factors as described hereinafter.

The present method and apparatus includes a conventional, rotatingscrew, extruder means and annular extrusion die means and conventionalinternal air, or other fluid, introduction means axially mounted in theannular die. The apparatus also includes conventional pinch roller meansspaced apart from the annular die a sufficient distance so that the tubepassing therethrough is sufficiently cooled to be substantially solidand non-tacky. Further conventional portions of the apparatus includemeans to provide sufficient air or other fluid inside the extrudedbubble to at least balance the ambient atmospheric pressure and permitthe extrudate to form the intended bubble.

One of the important features of the apparatus of this invention is theuse of means for imposing a particular configuration on the extrudedtube of thermoplastic material from the time it issues from the annulardie to the time, downstream thereof, at which it is more in the solid,substantially non-plastic state. This configuration imposition meanscomprises a housing of generally diverging, e.g. conical, paraboloid orother similar, shape. It is not intended by this characterization ofthis configuration imposition housing that such be limited to an exactgeometric reproduction of such mathematical functions as have beenmentioned. Rather these are given as illustrative of the general type ofconfiguration which is useful in this invention. It can be generallystated that this housing diverges at some rate from an initial, upstreamposition proximate to and concentric about the annular extrusion die toprogressively more downstream positions less proximate to but stillconcentric with the annular extrusion die.

The housing referred to above has channels therethrough each terminatingin an aperture on the surface thereof directed toward the extrudedtubing. Each row of apertures lies along a plane generally normal to theaxis of the annular extrusion die. Each row of channels and of coursetheir corresponding apertures, is associated with one next adjacent rowof channels and apertures so that such rows cooperate in pairs. Theindividual apertures in each of said pair of rows are streamwisedlydirected apart at a very wide angle, in fact preferably the widest anglepossible considering all other mechanical as will become apparent fromthis entire specification. The axes of these cooperating aperture andchannel pairs are positioned at a very small acute angle with respect tothe surface of the thermoplastic tube extruded from the referred toannular die. Although it is probably the most efficient configuration,the referred to channels need not be cylindrical in shape but can be aconverging nozzle or other configuration. It is only important thatthese channel pairs cause fluid flowing therethrough to divergepreferably to a maximum extent, upon emergence from each aperture pair.

Means are provided for forcing air or some other appropriate fluidthrough the channels and out the apertures generally toward the extrudedthermoplastic tubing. Outward passage means are also provided betweenthe aforementioned pairs of aperture rows for the air or other fluid toescape. Thus a circulatory system is provided forcing fluid from theoutside toward the extruded tube, passing the fluid along the extrudedtube and then passing the fluid back to the outside. Where the fluid isair, the atmosphere can be a suitable reservoir to pump from and to. Ifother fluid is used, a suitable reservoir and closed system can beprovided or the outward passage can simply vent the fluid to theatmosphere.

This fluid suitably cools the extruded tubing and solidifies it to asufficient extent to render it non-tacky and dimensionally stable. Givenjust this function, the tube of extruded thermoplastic material wouldfreely expand according to the pressure applied by the internal fluid asa function of the cooling rate applied and inherent liquid strength ofthe thermoplastic material in use. The apparatus and process of theinstant invention goes further than simply controlling cooling rate andthereby attempting to control rate and type of expansion of the extrudedtubing. The particular configuration of the apertures referred to abovecauses the fluid flowing out of them into a jet pump effect with respectto the space between cooperating aperture pairs whereby forming asignificant vacuum and thus drawing the extruded thermoplastic tubingtoward the configuration imposition means and causing the extrudedtubing to conform closely to the shape of this configuration impositionmeans where the two are juxtaposed. If the temperature and flow rate ofthe external fluid are adjusted in consideration of the particularthermoplastic being extruded and in consideration of the dimensions ofthe extruded tube as well as the annular extrusion die, the frost lineof the extruded tubing can be positioned within the configurationimposition means, generally toward the downstream end thereof.

It should be clear that many of the processing variables areinterdependent and are not independently definable. One purpose of thisinvention is to impose a particular rate and shape of expansion upon anextruded tube of thermoplastic material while cooling the extrudedmolten tube to a dimensionally stable and non-tacky condition. It hasbeen found that the shape imposed during such expansion and coolinggenerally is diverging and has a maximum total included divergence angleof less than about 60° preferably less than about 50°.

In another aspect of the present invention it has been found to bepreferred to form the individual pairs of aperture rows in a detachablering means which is mounted on the inner periphery of the air carryingplenum channels adjacent the advancing tubing. The detachable mountingof the ring facilitates ring removal for ease of cleaning the apertureswhich may become clogged with foreign materials during extrusionoperations. Additionally, since most of the intricate machining of theindividual air rings is done on this portion of the ring, the cost isminimized by minimizing the size of the piece being machined.

The thermoplastic material extruded into a tube according to thisinvention may be a polyolefin such as polyethylene, polypropylene,polybutene-1, copolymers of two or more of these with or without otherolefins, polyvinyl or vinylidene chloride, vinyl or vinylide chloridecopolymers with acrylates, acrylonitrile, olefins and the like, acrylichomo and/or copolymers, styrene homo and/or copolymers, and in general,such other polymeric materials are conventionally melt extruded intofilm form.

The thermoplastic is usually extruded through an annular die having adiameter of about 0.5 to 50 inches and a die gap of about 0.010 to 0.100inch. Conventional air ring cooling processes may operate with a givencombination of die orifice, blow-up ratio (ratio of bubble diameter todie diameter), extruder output and haul-off speed in order to produce afilm of a given thickness. In accordance with the present invention, ithas been found that converting such conventional conditions directly tothe herein described imposed-shape process will usually result ininoperability. It has been found that, using conventional conditions,the molten extrudate will become slack as it leaves the die, allowing itto fold over on itself or flow outward between the vertically spacedcooling ring elements, so that it hangs up, sticks, or in some waybecomes caught on the surfaces of the cooling ring elements immediatelyadjacent the annular die. It has been found that such inoperability canbe substantially reduced or eliminated by increasing die orifice gap toform a gap which is wider than would normally be employed inconventional tubular extrusion operations. By using this larger orifice,it has been found possible to utilize the present process. In aconventional cooling process, the usual gap may be in the range of 0.025to 0.040 inches, for example, whereas the required orifice gap in accordwith the process of the present invention is 0.040 to 0.050 inches. Forexample, a 26 inch lay-flat tubular width (52 inch circumference) film,0.00285 inches thick, is normally made with a 0.040 inch gap on aconventional prior art process, but requires a 0.050 inch gap with thepresent process. By using a larger die orifice gap, the molten film isthicker leaving the die, and must be drawn down with greater force toachieve desired final film thickness at the frost line. This greaterforce overcomes the tendency of the molten bubble to fold over on itselfor flow outward between the cooling ring elements as aforedescribed.Although the exact mechanism is not understood it is theorized that theenlargement of the orifice gap and proportionate increase in the drawdown force eliminates the problem of tube hang-ups, since as a result ofincreasing the draw down force a downward drag exerted by the coolingring elements on the molten film tube is overcome. For purposes of thepresent invention it has been found that a ratio of die orifice gap tofinal film thickness should be above about 10:1 and preferably aboveabout 15:1.

Extrusion rates are of course dependent upon the extruder used, however,flow rates of about 2 to 25, preferably 4 to 20, pounds per hour perinch of final bubble circumference can be easily maintained with theparameters of the practice of this invention. Blow up ratios, that is,the ratio of final film diameter to die diameter, on the order of about1.0 to 5 are suitable as are final film thickness of about 0.4 to 10mils. The preferred internal pressurizing fluid and external coolingfluids are air, however, other similarly acting relatively inert gasescan be used. The internal fluid should be maintained at about orslightly above atmospheric pressure. The external fluid should bemaintained at a temperature of about 0° to 200° F. and be fed at a rateof about 75 to 600 SCFM per square foot of surface area of the moltentubing being cooled. It is within the scope of this invention to providemeans within the island portion of the extrusion die means to extractsome or all of the air injected into the extruded tube so as to form aflowing system of air.

As shown in FIG. 1, there are provided a plurality of individual,hollow, cooling ring elements 34 which have centrally located circularopenings of varying diameter to allow for passage therethrough ofupwardly advancing thermoplastic tube 26. Each cooling element 34 hascircular openings 38 located around the edge of element 34. The circularopenings are superposed when the ring elements are in stacked alignmentthereby forming a common channel for the passage of fluid therethrough.As shown in FIG. 1 a pump 36 feeds a fluid, suitably air, into thecommon channels formed by superposed circular openings 38. This fluidenters chamber 32 of hollow cooling ring 34 through aperature slots 49located around the periphery of opening 38. This fluid is then impingedupon the extruded tubing 16 and 26 respectively through the divergingchannels 42a and 42b via apertures 43. Apertures 43 are machined intodetachable ring insert elements 40 which are detachably secured to theperiphery of the centrally located circular openings of the hollow,plate-like, cooling ring members 34. A plurality of screw-like members47 secure the insert elements 40 to cooling ring 34 as more clearlyshown in FIG. 5. The fluid impinging on tubing 16-26 creates a decreasedpressure between the aperture rows in the area designated 44. The fluidis allowed to pass out of the system through passages 46 betweenadjacent cooling ring members 34. Passages 46 are formed by theseparation between adjacent ring members 34. This separation is providedby the circular raised boss members 48 surrounding the upper and lowerperipheries of openings 38. When a plurality of cooling members 34 arearranged in superposed relation with their respective circular openings38 in vertical alignment, the circular boss members 48 maintainseparation between adjacent cooling ring members 34 and thereby providepassages 46 for fluid escape from the system after it has impingedagainst the surface of tube 16-26.

In accordance with a specific embodiment of the present invention theconfiguration imposing means is composed of a series of stacked,vertically spaced apart cooling rings of similar cumulative height. Therings are characterized by having a common air inlet chamber which isformed when the rings are superposed one upon the other. The two rows ineach pair should be spaced about 1/16 to 3/4 inch apart with the rowpairs spaced about 1/2 to 4 inches apart. The apertures themselves andthe channels related thereto should preferably be in radial planesrespectively and should be spaced apart about 2 to 6 aperture diametersapart in each row and the channel pairs should diverge about 50° to160°, preferably 100° to 150°. The air return means between aperture rowpairs should be about 1/8 to 1/2 inches wide so as to permit ready flowof the air back from the extruded tubing. It should be noted that thevelocity and/or temperature of the external air can be substantiallyconstant, at all aperture positions or it may be profiled in either orboth regards as processing conditions dictate.

Referring now to the drawings, and in particular FIG. 1, suitablethermoplastic resin 10 is fed to an extruder 12 in which it isplastified and then extruded through an annular die 22 into a tube 16 ofmolten thermoplastic material. Means 14 and 18, in the island 20 of theannular die, 22 are provided for feeding a fluid, suitably air, into thethermoplastic tube 16. The introduction of fluid into the bubble is asnecessary to maintain a controlled pressure within the bubble. As thetube proceeds downstream it cools until, at a frost line 24, itsolidifies into a dimensionally stable tubular structure 26. This solidtube 26 is collapsed by a guide 28 and then passes through the nip of apair of take off rollers 30 from whence it is taken to other processing(not shown) or wound into a roll.

The high velocity fluid traveling parallel to the film, in the areadesignated Y in FIG. 5, also causes a slight decrease in pressure inaddition to the pressure drop as aforedescribed in area 44. Thesepressure drops draw the still molten extruded tube 16 towards the rings34 but the ejecting fluid forms a cushion between the rings and thetubing so as to prevent the tubing from contacting the rings and gettinghung up on them while still in the molten condition.

Recesses 51 are provided in the sides of ring members 34 to accomodateelongated rods (not shown) which, when fastened to a group of rings 34,hold the stack of rings together as a unit. This facilitates removal ofthe upper group of rings (about the top half) in a single operation whenit is desired to change the width of the extruded tubing. Usually thelower group of rings, which may be similarly fastened together, mayremain in place adjacent the die during such a width changeover whilethe upper ring group is replaced by a stack of ring members havingeither larger or smaller centrally located openings dependent upon thedesired circumference of the extruded tubing.

Ring members 34, as shown in FIG. 3, may be provided with integralinternal rib members 50 to provide for overall stiffness of theindividual ring structures.

From the foregoing it will be apparent to those skilled in the art thatthis invention is capable of relatively wide modification withoutdeparture from its essential spirit, wherefor, it is intended to belimited only by the scope of the following claims.

What is claimed is:
 1. In an apparatus for forming tubular thermoplasticfilm comprising extruder means; annular extrusion die means, adapted incombination to extrude a seamless tube of thermoplastic film; pinchrollers spaced downstream from said annular extrusion die adapted toflatten said tube; and means for introducing a fluid under pressure intosaid tube to biaxially expand said tube and reduce the wall thicknessthereof; the improvement which comprises a configuration impositionassembly disposed concentric to said extruded tube downstream of andadjacent to said annular die, which assembly comprises a series ofspaced pairs of diverging aperture row means directed toward saidextruded tubing, means for forcing a fluid through said apertures towardsaid extruded tubing thereby creating a suction and thus drawing saidextruded tubing toward said assembly, said configuration impositionassembly further comprising a plurality of vertically spaced-apartindividual plenum chambers, said aperture row means being furthercharacterized by being detachably connected to said individual plenumchambers, and a common chamber formed by superposed individual plenumchambers for feeding said fluid under pressure into said individualplenum chambers.
 2. An apparatus in accordance with claim 1 wherein saidcommon chamber comprises a plurality of superposed openings saidopenings having raised bosses surrounding their upper and lowerperipheries.